Vehicles powered by internal combustion engines are typically equipped with exhaust after-treatment catalysts, filters, adsorbents, and other devices to comply with regulatory exhaust emission standards for carbon monoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NOx), particulate matter (PM), and so on. The effectiveness of exhaust after-treatment devices for removing the regulated emissions can vary with engine operating conditions, particularly temperature. Generally, there are minimum temperature requirements for successful implementation of after-treatment devices. These minimum temperature requirements are commonly referred to as “activation temperatures” for the devices. For the successful application of a given after-treatment device, the level of pollutant removal is usually not sufficient when the temperature is less than the activation temperature. In general, activation temperatures are dynamic in nature and vary depending on the after-treatment device, the application of the device, and the targeted pollutant removal.
For example, FIG. 1 shows the impact of temperature on oxidation catalyst efficiency of one catalyst that illustrates behavior typical for commercially used catalysts. As temperature increases the oxidation efficiency increases dramatically until it plateaus at a relatively high value. The temperature at which the catalyst's efficiency begins to plateau at the relatively high value is referred to as the catalyst's activation temperature. The catalyst of FIG. 1 has an activation temperature of about 200° C., where its efficiency is about 80%. (This intersection is shown as point 100 in FIG. 1.) After about 200° C., increases in catalyst efficiency are gradual and modest. The increase in efficiency before reaching the activation temperature is sharp; the example illustrated in FIG. 1 has an increase from about 10% efficiency to about 80% efficiency (an eight-fold increase in efficiency) between 150° C. and its activation temperature of 200° C. Thus, small decreases in temperature below the activation temperature of an after-treatment device can result in a significant reduction in the efficiency of the device for treating the regulated emissions in the exhaust stream.
During vehicle operation, the exhaust gas temperature varies depending on factors such as the engine speed, load, and controls change with driving conditions. FIG. 2 shows the exhaust gas temperature for a vehicle operating on a given driving cycle. Vehicle acceleration increases the engine load, which results in increased exhaust gas temperature. Generally, exhaust gas temperatures are maintained as the vehicle cruises at highway speeds. During vehicle deceleration and idle conditions, however, the exhaust gas temperature rapidly decreases. Other driving conditions, such as driving at low speeds in winter weather, lead to low exhaust gas temperatures.
The driving schedule and engine-vehicle system, then, together can result in periods during which the exhaust gas temperatures are below the activation temperatures needed for the after-treatment devices used on the vehicle to achieve the desired emissions reduction. For example, the oxidation catalyst shown in FIG. 1 has an activation temperature of about 200° C. The exhaust gas temperature shown in FIG. 2, however, is frequently below 200° C. At the lower temperatures, the regulated emissions elimination efficiency, or “conversion efficiency,” of the oxidation catalyst will drop below the optimum, desired level. In the example of FIG. 1, the desired conversion efficiency level is at least 80%. If the catalyst were used for treating exhaust from a vehicle that generated exhaust gas temperatures as illustrated in FIG. 2, the exhaust gas would be at temperatures below the activation temperature of the catalyst a significant fraction of the time. In the right-hand third of the graph, exhaust gas temperatures that are often from about 150° C. to about 160° C. provide catalyst efficiencies of less than 20%. Under such circumstances, additional steps must be taken to achieve the desired level of emissions removal.
It would, therefore, be desirable to develop technologies that control the exhaust gas flow rate and temperature to achieve and maintain the activation temperatures of the after-treatment devices.